The present invention relates to an apparatus and a method for alleviating or reducing shocks occurring in effecting a shift in an automatic transmission.
Automotive automatic transmissions are known wherein an appropriate gear is automatically selected for vehicle speed and engine load (throttle opening degree, for example) in accordance with a predetermined shift schedule or pattern and an upshift or a downshift to the selected gear is effected for smooth running of the vehicle.
Commonly, the automatic transmission effects a shift with engine load (engine throttle opening, for example) unchanged. Particularly, an upshift is effected with the throttle kept open, viz., with an accelerator pedal kept depressed. The fact that the upshift is effected with the acclerator pedal depressed causes the occurrence of substantial shocks in effecting the upshift. Referring to FIG. 11, there is illustrated how substantial shocks are produced in effecting a 2-3 upshift. It will be readily understood from FIG. 11 that the engine rapidly drops its revolution speed during a period from t.sub.1 to t.sub.2 as indicated by phantom line x and at the same time the transmission output shaft torque has a peak as indicated by phantom line y. This torque peak which is higher by T.sub.1 than the level assumed by the transmission output torque after the completion of the shift, causes occurrence of substantial shocks. Similarly, in the case of effecting a 1-2 upshift as illustrated in FIG. 13, the engine rapidly drops during a period from t.sub.3 to t.sub.4 as indicated by phantom line x' and at the same time the transmission output shaft torque has a peak as indicated by phantom line y'. This peak is higher by T.sub.2 than the level which the transmission output shaft torque assumes after completion of the shift, and this torque peak causes the occurrence of substantal shocks.
The peak torque y or y' is caused by a rapid change in torque that is transmitted to the trasnmission output shaft due to a rapid change in the engine revolution speed as indicated by the phantom line x (in FIG. 11) or x' (in FIG. 13). The magnitude of the peak torque increases in proportion to an increase in engine load (throttle opening degree or depression degree of accelerator pedal, for example).
In order to suppress the peak torque as discussed above, published Japanese Pat. No. 46-2466 discloses a measure wherein fuel supply to the engine is restrained in effecting an upshift so as to cause a drop in the engine torque. This measure, however, is not satisfactory because a drop in engine revolution speed due to temporal shortage in fuel supply causes shocks or vibrations of the vehicle body.
Therefore, an object of the present invention is to provide a shift shock alleviating apparatus and method for an automatic transmission which is free from the above mentioned problems.